1. Field of the Invention
This invention relates generally to a front/back porch voltage-regulator varying a front/back porch voltage level of a vertical focus control signal in the displayer such as televisions or monitors using a cathode-ray tube as a displaying equipment.
2. Description of the Prior Art
A cathode-ray tube is generally composed of an electron gun generating a red, green and blue electron beam at a back neck, a shadow mask and, in front, red, green and blue phosphor for emitting red, green and blue light when struck by the beam for expressing a certain image. The red, green, and blue electron beam generated by the electron gun pass through an electron beam passing hole before colliding with the red, green and blue phosphor. Accordingly, the image can only be expressed exactly and clearly when the red, green and blue electron beam generated by the electron gun collides exactly with the red, green and blue phosphor. Therefore, a focus electrode must be equipped to regulate the focus of the electron beam generated by the electron gun, and a certain focus regulating signal must be applied to the focus electrode.
The focus electrode generates a certain electric field according to the applied level of the focus regulating signal, and regulates the focus by regulating the width of the electron beam according to the intensity of the generated electric field. Since the distance from the electron gun to the middle, the upper, lower, left and right portions of the screen differ from each other, the level of the focus regulating signal must be varied according to the colliding position of the electron beam to the phosphor, that is the deflected position. As a result, a display using a cathode-ray tube as a displaying equipment generates horizontal and vertical focus regulating signals according to the frequency of the horizontal and vertical synchronous signals.
U.S. Pat. No. 4,555,649 to Gulab Sharma and entitled Cathode Ray Tube Focusing Circuitry, contemplates focus control by taking a sawtooth wave signal from a vertical deflection circuit and integrating it in an operational amplifier to produce a parabolic wave signal which is applied to the inverting input of a second operational amplifier. The inverted parabolic wave signal is then added to another parabolic wave signal applied to the other input of the second operational amplifier, which was produced by a S-correction capacitor of a horizontal deflection circuit, to produce a parabolic signal envelope. The parabolic signal envelope is then inverted, amplified and applied as a dynamic focus signal to the focus electrode of the cathode ray tube to modulate the focus electrode's static voltage.
U.S. Pat. No. 5,512,964 to Kang H. Kim entitled Dynamic Focusing Circuit Having A Psuedo Horizontal Output Circuit To Eliminate Phase Deviation In A Focus Signal contemplates enhancing the correctness of the focus by use of a plurality of variable resistors which can control the amplitude of the vertical and horizontal components of the dynamic focus signal, the DC level of the dynamic focus signal, and the phase deviation between the horizontal deflection component and the horizontal component of the dynamic focus signal.